Unfortunately, HP does not currently offer a way to change the font/size of the X/Y-Axis value labels (you can only change the font/size of the title), but here are a couple pointers:
1. When you're looking at your Graph in Analysis View, click on the View tab. Within the View tab, click on the ''Set Graph Appearance'' button. In the Appearance tab, UNCHECK the ''3D Graph'' option. This will force a 2D perspective that will give more room to your X/Y-Axis values.
2. When creating your Dashboard View and adding your Pie Graph, make sure you expand it so that it's not being shared by another graph on the same page (Dashboard View -> Select your Page -> Configuration Tab -> Remove any adjacent graph entity, if applicable -> Click on the upper-left button of the Pie Graph entity to Expand). This will give your Pie Graph more room to display the X/Y-Axis labels (you can also select ''View Graph in Full Screen'' when looking at the graph in your View tab).